Edit: Literally the only way to get this to work properly is to make add-on developers upload their content and manage it here. I’d love that from a UI standpoint and am sure it would assist sales, but that’s increasing burden significantly on XF and more importantly some developers (ThemeHouse springs to mind, ourselves as well) have invested in custom panels to deliver a better experience. The panel on nanocode.io actually complies with various laws too, namely VAT MOSS and issues compliant invoices. There’s way too much this site would need to do for legal compliance to make that worth it, and even then some developers would be angry they’ve invested time in their panels that they can no longer use.
So you’re stuck to the problem where add-ons are hosted externally and we go back to problem 1.
Honestly, certification is the only real solution I see here.